1. Field of the Invention
The present invention relates to a gate driving circuit and related liquid crystal display (LCD) device, and more particularly, to a gate driving circuit and related LCD device capable of separating time for each channel to turn on a thin film transistor (TFT), in order to facilitate dispersing current when the LCD device is turned off.
2. Description of the Prior Art
A liquid crystal display (LCD) device has merits such as light weight, low power consumption, and low radiation, and therefore has been widely used in information products, e.g. a computer system, a mobile phone, a personal digital assistant (PDA). Operating principles of the LCD device are that different orientation of liquid crystal molecules has different polarization and refraction effects to light beams. Thus, light transmittance of the LCD device can be controlled by altering the orientation of the liquid crystal molecules, so as to generate light with different intensity, and red, blue and green lights with different gray levels.
Please refer to FIG. 1, which is a schematic diagram of a conventional thin film transistor (TFT) LCD device 10. The LCD device 10 includes an LCD panel 100, a timing control circuit 102, a source driving circuit 104, a gate driving circuit 106 and a common voltage generator 108. The LCD panel 100 includes two substrates, and liquid crystal molecules are filled between these two substrates. One substrate is disposed with a plurality of data lines 110, a plurality of scan lines (gate lines) 112 perpendicular to the data lines 110, and a plurality of TFTs 114, while the other substrate is disposed with a common electrode for providing a common voltage Vcom via the common voltage generator 108. For the sake of simplicity, only four TFTs 114 are shown in FIG. 1, but in practical, there is one TFT 114 at every intersection of each data line 110 and scan line 112, i.e. the TFTs 114 are disposed on the LCD panel 100 in matrix. Each data line 110 is corresponding to a column of the LCD device 10, each scan line 112 is corresponding to a row of the LCD device 10, and each TFT 114 is corresponding to a pixel. Besides, the circuit characteristics of the two substrates of the LCD panel 100 can be seen as an equivalent capacitor 116.
In the LCD device 10, the timing control circuit 102 generates and outputs control signals to the source driving circuit 104 and the gate driving circuit 106 respectively, and thus, the source driving circuit 104 and the gate driving circuit 106 generate input signals for different data lines 110 and scan lines 112, so as to control conduction of the TFTs 114 and voltage difference of the equivalent capacitor 116, and further alter the orientation of the liquid crystal molecules and the corresponding light transmittance, to show image data 122 on the LCD panel 100. For example, the gate driving circuit 106 inputs a pulse into the scan lines 112, to conduct the TFTs 114. Therefore, signals inputted into the data lines 110 by the source driving circuit 104 can be inputted into the equivalent capacitor 116 via the TFTs 114, so as to control the gray level status of the corresponding pixel. In addition, different gray levels can be generate by controlling magnitude of signals inputted into the data lines 110 via the source driving circuit 104.
Since circuit characteristics of the liquid crystal is similar to a capacitor, the equivalent capacitor 116 stores charges with different coulombs during operations of the LCD device 10. If the charges stored in the equivalent capacitor 116 are not effectively released when the LCD device 10 is tuned off, the LCD panel 100 generates phenomena of residual images, blinking, etc, affecting image quality when the LCD device 10 is turned on again. Therefore, in order to solve the above problems, the conventional LCD device 10 needs a mechanism for releasing residual charges when the LCD device 10 is turned off, which is detailed as follows.
Signals outputted from the timing control circuit 102 to the gate driving circuit 106 include a shutdown indication signal XON, which is utilized for indicating an operation state of the LCD device 10. For example, when the shutdown indication signal XON is at a high level, the LCD device 10 is in an ON state, and when the shutdown indication signal XON is at a low level, the LCD device 10 is in an OFF state. Therefore, when the LCD device 10 is turned on and not yet turned off, the shutdown indication signal XON is still at the high level. When the LCD device 10 is turned off by a user or a system control, the level of the shutdown indication signal XON shifts to the low level immediately. When the level of the shutdown indication signal XON shifts from the high level to the low level, the gate driving circuit 106 outputs a high voltage level voltage VGH to each channel (i.e. the scan line 112), to turn on all the TFTs 114, such that the residual charges of the equivalent capacitor 116 can be released, so to avoid phenomena of residual images, blinking, etc. when the LCD device 10 is turned on again.
When all channels output the high voltage level voltage VGH, which can be seen as all channels simultaneously drain currents from a power supply, a voltage drop occurs when the currents pass conductive wires, such that operating timing of the gate driving circuit 106 is affected, leading to abnormal display. In order to avoid the above problems, a proper delay is generated in the transmission path of the shutdown indication signal XON in the prior art, to separate time for each channel to output the high voltage level voltage VGH, for dispersing current supply. Generally, methods for generating a delay utilize resistors/capacitors (RC) circuits, i.e. a transmission path of the shutdown indication signal XON between neighboring channels is set by an RC circuit, for delaying the shutdown indication signal XON. However, RC circuits have high variations and cannot generate a uniform time constant, causing too less or too much delay, which affects charge releasing operation and even results in abnormal display.